The present invention relates generally to an apparatus for suppressing a knocking phenomenon in an internal combustion engine (hereinafter also referred to as an engine for short) such as a gasoline engine for a motor vehicle.
In general, the internal combustion engine such as a gasoline engine for a motor vehicle includes a plurality of cylinders in each of which a fuel gas mixture is compressed and undergoes combustion at an optimal timing. In this conjunction, there has already been proposed and used widely in practical applications a microcomputer-based engine control unit (also known as ECU in abbreviation) for the purpose of optimally controlling the ignition timing as well as the sequence of fuel injections in association with the individual engine cylinders.
In connection with such engine operation control, it is known that when the ignition timing (usually given in terms of angular crank position or crank angle) is controlled to advance excessively, abnormal fuel combustion may take place, resulting in generation of vibrations or shock referred to as knocking of such a magnitude which may eventually damage or injure the engine cylinders. In order to avoid such an unwanted event, it is necessary to perform ignition timing control in such a manner that upon detection of abnormal vibrations or knocking, the ignition timing is shifted in a direction to afford an appropriate retard to the time point or timing at which fuel combustion takes place within the knocking cylinder.
For a better understanding of the background of the present invention, a known knock suppressing apparatus will be described in some detail by reference to FIG. 8 which is a block diagram showing the general arrangement of a known knock suppressing apparatus.
In FIG. 8, a reference numeral 1 denotes a knock sensor installed in association with each or a set of the cylinders of an internal combustion engine. The knock sensor 1 may be composed of a piezoelectric element or the like which is capable of detecting the vibrations or knocking of the associated cylinder in the form of an electric signal.
An output signal A of the knock sensor 1 is supplied to a knock detection circuit denoted generally by a reference numeral 2. The knock detection circuit 2 comprises a filter 21 having such a filtering characteristic as to pass therethrough only the frequency components which are peculiar to the knocking phenomenon (e.g., 7 kHz), a gate 22 for allowing the output signal of the filter 21 to pass therethrough periodically at a predetermined timing, a background level (BGL) generator 23 for generating a background level signal BGL on the basis of a signal derived by averaging an output signal A' of the gate 22, a comparator 24 for comparing the output signal A' of the gate 22 with the background level signal BGL for thereby producing an output signal of "ON" level when the gate output level A' exceeds the background level BGL, and an integrator 25 for integrating the output signal of the comparator 24. The output signal of the integrator 25 is then supplied to an analogue to digital (A/D) converter 3 to be converted to a digital signal V.sub.R.
The digital signal V.sub.R is supplied to an engine control unit (ECU in abbreviation) 4 which may be constituted by a microcomputer which is programmed to perform ignition timing control for each of the engine cylinders on the basis of the output signal V.sub.R of the A/D converter 3 while supplying a masking pulse signal M to the gate 22 and a reset signal R to the integrator 25, respectively, for the purposes which will be described hereinafter. Further, the engine control unit or controller 4 includes a retard angle controller 45 for arithmetically determining an angle of retard for which the ignition timing is to be delayed for suppressing the knocking, thereby producing a retard control angle signal O.sub.R for controlling the amount of retard to be applied to the ignition timing on the basis of the digital signal V.sub.R outputted from the A/D converter 3.
Next, referring to a waveform diagram shown in FIG. 9, description will be made of operations performed by the known knock suppressing apparatus shown in FIG. 5.
Normally, in each of the cylinders of the internal combustion engine, ignition takes place at a timing corresponding to a crank angle or position which advances approximately by 5.degree. relative to top dead center (TDC given by the crank angle of 0.degree.) so that explosive combustion of the fuel gas mixture may occur at a crank angle of about 10.degree. to 60.degree. after passing top dead center (TDC). The knocking due to abnormal combustion will thus occur at the timing falling within the crank angle range of 10.degree. to 60.degree. in succession to top dead center.
Accordingly, upon every occurrence of vibration noise of the cylinders and inter alia knocking, the output signal A of the knock sensor 1 produced at corresponding periodical intervals assumes a significantly increased amplitude, as can be seen in the waveform shown in FIG. 9 at (a).
In the meanwhile, the engine control unit (ECU) 4 outputs to the gate 22 a masking pulse signal M which is inverted periodically at predetermined intervals in order to ensure that the knock detection circuit 2 can efficiently receive and process the sensor output signal A. More specifically, the masking pulse signal M is generated in such a waveform in which the leading edge thereof takes place at a time point corresponding to a crank angle of about 75.degree. advancing relative to the top dead center of the associated cylinder (this advanced angle will hereinafter be represented by affixing "B" to the angle value, e.g. by "B75.degree.") while the trailing edge of the masking pulse M occurs around a time point of B5.degree. (i.e. at a time point corresponding to a crank angle of 5.degree. before TDC), as can be seen in the waveform shown at (b) in FIG. 9. During the period in which the masking pulse assumes the level of "H", the gate 22 is blocked or disabled. Further, as mentioned previously, a reset signal R is outputted to the integrator 25 from the engine control unit 4 periodically at a predetermined timing which coincides with that of the leading edge of the masking pulse signal M.
The filter 21 constituting a part of the knock detection circuit 2 has such a filtering characteristic that the frequency components of the sensor output signal A produced upon occurrence of knocking can pass therethrough, while the gate 22 allows the sensor output signal A to pass therethrough only during a period in which the masking pulse signal M is at the level of "L", as shown at (c) in FIG. 9. The output of the gate 22 is denoted by a reference symbol A'. On the other hand, the background level (BGL) generator 23 generates a background level BGL contained in the gate output signal A' by discriminatively separating the former from the latter, as is illustrated at (d) in FIG. 9, wherein the background level BGL serves as a reference for detection of a knocking event or phenomenon.
When the gate output signal A' exceeds the background level BGL, the comparator 24 determines that knocking has taken place and produces a comparison output of "H" level. The integrator 25 starts to integrate the output signal of the comparator 24 every time it is reset by the reset signal R supplied from the engine control unit 4, as is illustrated at (e) in FIG. 9. The output signal of the integrator 25 is then converted form analog into digital form by the A/D converter 3, the resulting digital integration value V.sub.R being then inputted to the engine control unit (ECU) 4.
In this manner, the engine control unit 4 fetches therein the A/D converted integration value V.sub.R upon every occurrence of ignitions in the cylinder, to thereby generate a retarded control angle signal .theta..sub.R for controlling the ignition timing of a knocking cylinder in a sense to suppress the knocking. To this end, the retard angle calculator 45 constituting a part of the engine control unit 4 adds an angle of retardation d.theta..sub.R to a current normal ignition control angle .theta..sub.R *, at which ignition is to take place when there is no knocking, to provide a current retarded control angle signal .theta..sub.R. Accordingly, the current retarded control angle .theta..sub.R can be given by the following equation: EQU .theta..sub.R =.theta..sub.R *+d.theta..sub.R ( 1)
In equation (1) above, the angle of retardation d.theta..sub.R is given by the following equation: EQU d.theta..sub.R =V.sub.R .times.L
where L represents a weighing coefficient.
As will be understood from the foregoing, with the known knock suppressing apparatus as described above, the background level BGL, which is directly calculated based on an average of the output signal from the gate 22 in a predetermined period, is utilized as a threshold for knock determination. As a result, the threshold calculated in this manner always has a characteristic related to the average of the gate output, so it is difficult to arbitrarily obtain a desired knock determining threshold as necessary irrespective of the averaged gate output.